The present invention generally relates to synchronizing signal generating systems, and more particularly to a synchronizing signal generating system for a laser scanner.
A laser scanner uses a laser beam to write (record) and/or read information on and/or from a recording medium. Generally, the laser scanner is provided with a deflector such as a polygonal mirror (or polygonal scanner) for deflecting a laser beam which is to scan the recording medium. However, it is virtually impossible to keep the scan timing constant for each scan because the rotation of the deflector cannot be maintained perfectly constant and mirror surfaces of the deflector cannot be finished to perfect mirror surfaces. For this reason, a synchronizing signal is required to control the scan timing to an optimum timing.
Conventionally, prior to each scan by the laser beam, the scan is synchronized by detecting the laser beam immediately prior to the scan. However, such a synchronization which simply detects the laser beam at one point prior to each scan is insufficient, because the scanning speed is not perfectly constant due to a deviation in the rotational speed of the deflector, a deviation of the characteristic of a f.theta.-lens from an ideal linear characteristic and the like.
Accordingly, methods of more accurately synchronizing the scan were proposed in Japanese Laid-Open Patent Applications Nos. 54-97050 and 60-124938. These methods use a laser beam for recording and another laser beam for scan synchronization.
FIG. 1 shows an essential part of a recording apparatus employing a laser scanner in which the synchronizing signal is basically generated according to such proposed methods. The recording apparatus comprises a laser diode 1 for emitting a laser beam for recording (hereinafter referred to as a recording beam, a laser diode 2 for emitting a laser beam which is used for generating a synchronizing clock signal (hereinafter referred to as a synchronizing beam), a polygonal mirror 3, an f.theta.-lens 4, mirrors 6 and 7, a concave mirror 8, a light receiving element 9 such as a photodetector, a grating 10, an amplifier 11, a shaping circuit 13, a laser diode driver 15, and an information source 17.
The laser beams emitted from the laser diodes 1 and 2 are deflected by the polygonal mirror and is transmitted through the f.theta.-lens 4, so that the recording beam scans a recording medium 5 such as a photosensitive sheet or drum to record an information and the synchronizing beam scans the grating 10 by way of the mirrors 6 and 7.
As shown in FIG. 2, the grating 10 comprises minute bright portions 10a and minute dark portions 10b which occur alternately with a predetermined pitch. When the synchronizing beam scans the grating 10 and a beam spot SP of the synchronizing beam moves in a direction A, the intensity of the synchronizing beam transmitted through the grating 10 becomes modulated depending on the arrangement of the bright and dark portions 10a and 10b. The synchronizing beam transmitted through the grating 10 is converged by the concave mirror 8 and is directed to the light receiving element 9 where it is subjected to a photoelectric conversion. The light receiving element 9 outputs a pulse signal which is passed through the amplifier 11 and the shaping circuit 13, and an output pulse signal of the shaping circuit 13 is supplied to the laser diode driver 15 as a synchronizing clock signal. The laser diode driver 15 produces an image clock signal which has a frequency higher frequency than that of the incoming synchronizing clock signal and is synchronized to the synchronizing clock signal, and drives the laser diode 1 in synchronism with the image clock signal depending on information data entered from the information source 17. Since the synchronizing clock signal is generated based on the synchronizing beam, the driving timing of the laser diode 1 is automatically adjusted even when the rotational speed of the polygonal mirror 3 becomes unstable during the recording operation. Therefore, the recording operation is carried out with an appropriate scan timing.
The grating 10 itself is known, and is sometimes referred to as a slit or grid scale. The slit scale comprises a light transmitting portion and a non-transmitting portion which occur alternately with a predetermined pitch.
The problem of the conventional methods of generating the synchronizing signal is in that the synchronizing beam transmitted through the grating 10 is converged and directed to the single light receiving element 9 by use of the concave mirror 8. In other words, when the scanning distance (width) per scan becomes long, it becomes necessary to use a large concave mirror, but such a large concave mirror cannot converge the synchronizing beam satisfactorily to a small beam spot on the light receiving element 9 due to aberration and errors introduced during the production of the concave mirror. As a result, it becomes extremely difficult to generate the synchronizing clock signal with a high accuracy when such a large concave mirror is used.
The use of a mirror array is proposed in a Japanese Laid-Open Patent Application No. 60-72473 as a method of eliminating some of the problems described before. According to this method, the synchronizing beam transmitted through the grating is converged and directed to a plurality of light receiving elements by a plurality of concave mirrors constituting the mirror array. However, a vapor deposition process is needed to produce such a mirror array, and the production cost of the mirror array is high. In addition, since the plurality of light receiving elements are located in an optical path between the grating and the mirror array, the mirror array must be arranged obliquely, that is, optical axes of the plurality of concave mirrors of the mirror array must lie on a plane oblique to an optical axis of an f.theta.-lens through which the beam reaches the grating, so as to avoid interference of the beam directed to the mirror array and the beam directed to each of the plurality of light receiving elements, and the positioning of the mirror array is difficult and troublesome to perform.
In addition, scattering of the reflected light occurs at each boundary portion between two mutually adjacent concave mirrors of the mirror array, and there is a decrease in the quantity of light reaching the light receiving element from the boundary portion. As a result, the output of the light receiving element deviates and the synchronizing clock signal becomes unstable, thereby making it impossible to carry out an accurate synchronous detection. In order to ensure the generation of a stable and accurate synchronizing clock signal, it is essential to provide a compensation circuit to compensate for the output deviation of the light receiving element caused by the scattering of the reflected light at the boundary portion, but the use of such a compensation circuit makes the construction of the laser scanner complex.
In other words, the duty cycle of the synchronizing clock signal becomes unstable at the boundary portion between the two mutually adjacent concave mirrors of the mirror array. In extreme cases, a signal dropout occurs at the boundary portion. Usually, a phase locked loop (PLL) circuit is used to match the phase of the image clock signal with that of the synchronizing clock signal. The image clock signal is used to enable and disable the recording operation. The PLL circuit comprises a phase comparator for comparing the phases of the synchronizing clock signal from the shaping circuit and an output signal of a voltage controlled oscillator (VCO) which is controlled by an output control voltage of the phase comparator, and the output signal of the VCO is used as the image clock signal.
For this reason, the unstable duty cycle of the synchronizing clock signal and the signal dropout in the synchronizing clock signal cause the PLL circuit to run from the locked state and cause a sudden change in the oscillation frequency of the VCO. In these cases, the image clock signal becomes unstable and deteriorates the quality of the recording made on the recording medium. In terms of the freqency, the change in the duty cycle of the synchronizing clock signal causes a voltage change in the output control voltage of the phase comparator, and this change in the control voltage causes a frequency change in the image clock signal. The change in the frequency of the image clock signal appears as moire and the like on the recording medium and greatly deteriorates the quality of the recording.
On the other hand, a Japanese Laid-Open Patent Application No. 60-75168 discloses a method of eliminating the undesirable effects of the scattering of the reflected light at the boundary portion. This method uses two mirror arrays which are essentially positioned one on top of the other. A first mirror array is made up of concave mirrors having boundary portions which do not coincide with boundary portions of concave mirrors constituting a second mirror array. A first group of light receiving elements are provided to receive reflected lights from the first mirror array, and a second group of light receiving elements are provided to receive reflected lights from the second mirror array. The synchronizing clock signal is derived by adding outputs of the light receiving elements in the first and second groups. However, this method requires a complex converging optical system, and furthermore, a light receiving system for receiving the converged light from the converging optical system also becomes complex due to the large number of light receiving elements used.